Although C-arm fluoroscopy is widely used for visual assessment during clinical procedures, a limitation is that it cannot show soft tissues or organs of interest and other relevant structures. On the other hand, use of other imaging modalities such as ultrasound during clinical procedures provides real-time visualization of the tissue/organ of interest but does not provide radiographic information. Fusion of these complementary imaging modalities would provide benefits during many procedures.
For example, interventional tool registration with soft tissue would benefit from being able to accurately view the tool and the soft tissue of interest simultaneously during a medical procedure. The procedure could be performed with greater precision through improved tool visualization and registration with the soft tissues undergoing the procedure. Further, the ability to visualize the soft tissue using another imaging modality such as ultrasound provides real time information to the clinician performing the procedure.
Brachytherapy is such a procedure. Brachytherapy involves the placement of radioactive pellets or “seeds” into or adjacent cancerous tissue of a patient. Brachytherapy makes it possible to treat the cancer with a high total dose of radiation in a concentrated area in a short period of time, and at the same time spare healthy tissues the treatment with radiation. The key to successful brachytherapy is the accurate placement of the seeds. However, faulty needle and seed placement often cause an insufficient dose to the cancer and/or inadvertent radiation of healthy tissues. The ability to perform dosimetry optimization during the procedure could change the standard of care in brachytherapy, but such function is not available today and it is unfortunate that implants are currently performed without an explicit dosimetry evaluation in the operating room. Generally, dosimetric analysis requires precise localization of the implanted seeds in relation to the cancerous tissue and surrounding anatomy.
Brachytherapy may be performed with ultrasound guidance that provides real-time visualization of the tissue/organ of interest but not of the implanted seeds for a variety of reasons, including but not limited to scattering from implanted seeds, noise in the image, and shadowing caused by implanted seeds. C-arm fluoroscopy, on the other hand, is widely used for gross visual assessment of the implanted seeds but it cannot sufficiently image the soft tissue/organ and other relevant structures. Fusion of these complementary modalities would enable dynamic dosimetry, wherein the dose distribution is updated dynamically based on the actual positions as the seeds are deposited. For fusion of these modalities, 3D reconstruction of the seeds is necessary and requires that the relative poses of the fluoroscopy images must be known prior to reconstruction.
Pose recovery on C-arm fluoroscopy machines is a major technical problem that presently does not have a clinically practical solution in many areas of application. Currently, the relative poses of fluoroscopy images are determined in one of following three ways: (i) electronic joint encoders, (ii) optical or electromagnetic tracker, and (iii) radiographic fiducials. Fully encoded C-arm fluoroscopes are very expensive and thus virtually non-existent in brachytherapy. External trackers are also impractical for various reasons and also add costs. Optical tracking (e.g., VectorVision® Navigation System, “Brainlab, Inc., Heimstetten, Germany; StealtStation®, Medtronic Surgical Navigation Technologies, Louisville, Colo., USA) requires line of sight which imparts alterations in clinical setup and workflow. Electromagnetic tracking (e.g., OEC 9800 FluoroTrak™, GE Healthcare, Waukesha, Wis., USA) overcomes these issues, but it is susceptible to field distortion from metal objects, such as the C-arm itself, and thus compromise on accuracy.
Several researchers have explored fiducial-based radiographic tracking [1-4]. In an effort to make fiducials better integrated in the clinical setup, compact fiducials have been explored. However, decreasing the size of the fiducial fixture also decreases tracking accuracy. In fiducial structures made up of beads, 1-3 mm translation accuracy and 1°-2° orientation accuracy in tracking the C-arm has been achieved [2-4]. For prostate brachytherapy Jain et al. developed a fluoroscope tracking (FTRAC) fiducial [5] and validated the device clinically [6]. The fiducial used spherical beads and straight lines and ellipses that are invariant to projection, in that they project as straight lines and ellipses (see FIG. 1B). However, the technique involved segmentation of different features of the FTRAC fiducial in C-arm images, which was found to be fragile in actual field practice [6].